Heat olive oil and butter in a large Dutch oven or heavy-bottomed pot over medium-high heat.
Add the sliced mushrooms and sauté until they have released their moisture and turned golden brown, approximately 10 minutes.
Add the onion, carrots, and celery to the pot. Sauté for 5-7 minutes until the onions are translucent.
Stir in the minced garlic and dried thyme; cook for 60 seconds until fragrant.
Add the rinsed pearl barley and stir to coat in the oil and aromatics.
Pour in the vegetable broth and soy sauce. Bring the mixture to a boil.
Reduce heat to low, cover the pot, and simmer for 40 to 45 minutes, or until the barley is tender.
Season with salt and black pepper. Stir in fresh parsley before serving.